1. Field of the Invention
The present invention relates to mounting structures fabricated by surface mount technology (hereinafter, will be abbreviated to SMT).
2. Related Art of the Invention
A mounting structure of the related art is fabricated in SMT steps as follows: first, solder paste composed of solder of spherical solid metals and liquid flux is applied to substrate electrodes on a surface of an insulating substrate by mask printing; an electronic component, e.g., a chip capacitor or an IC with electronic component electrodes is mounted on the solder paste; the solder is melted by heating to at least the melting point of the solder; and finally, the solder is solidified by cooling to electrically connect the substrate electrodes and the electronic component electrodes.
Unfortunately, solder balls may occur in the process of fabricating the mounting structure according to SMT.
FIG. 12 is an enlarged view of a principle part of the mounting structure having a solder ball. In FIG. 12, reference numeral 21 denotes substrate electrodes, reference numeral 23 denotes a resist, reference numeral 25 denotes an electronic component, reference numeral 26 denotes electronic component electrodes, reference numeral 27 denotes solder paste composed of flux and solder, and reference numeral 29 denotes a solder ball.
As shown in FIG. 12, the solder ball 29 is spherical solder and appears beside the electronic component 25 on the resist 23. The solder ball 29 occurs in the case where the solder paste 27 printed on the substrate electrodes 21 is pressed out onto the resist 23 under the electronic component 25 when the electronic component 25 is mounted, or solder particles flow under the electronic component 25 because flux containing the solder particles flows due to heat (hereinafter, will be referred to as a dipping phenomenon of flux) during heating.
The solder ball 29 is not metallically joined to the substrate electrodes 21 and the electronic component electrodes 26 and thus may fall off the electronic component 25 in the event of a dynamic load. The solder ball 29 having fallen off the electronic component 25 may move between the two substrate electrodes electrically unconnected to each other and cause a short circuit, resulting in failures such as malfunctions and breakdowns of electronic equipment. Particularly, in the case of an insulating substrate for vehicles, a dynamic load of vibrations during operations is large and the solder ball 29 is likely to occur. Thus it is important to suppress the occurrence of the solder ball 29 in view of safety and operation reliability of equipment.
In a mounting structure designed to suppress the occurrence of solder balls according to the related art, a flow of solder paste under an electronic component is suppressed when the electronic component is mounted (e.g., see Japanese Patent Laid-Open No. 2002-359461).
FIG. 13(A) is a side view showing the structure of the mounting structure described in Japanese Patent Laid-Open No. 2002-359461. FIG. 13(B) is a top view of FIG. 13(A).
The mounting structure of FIGS. 13(A) and 13(B) includes an insulating substrate 20 having an electronic component 25, electronic component electrodes 26, solder paste 27, and substrate electrodes 21.
In FIGS. 13(A) and 13(B), under the electronic component electrodes 26, the opening of a metal mask is formed such that an edge 27a of the solder paste 27, that is, an edge of the opening of the metal mask (not shown) is located inside an edge 21a of the substrate electrode 21. Thus the occurrence of solder balls is suppressed by reducing the amount of the solder paste 27 pressed out of the substrate electrodes 21 when the electronic component 25 is mounted, without considerably reducing the total amount of the solder paste 27.